Polyatomic Molecules

We use polyatomic molecules to realize quantum control via laser cooling and trapping while maintaining large polarizability and properties favorable for precision measurements.

Searching for New Physics at the PeV Scale

Searches for new physics with molecules typically involve measuring phases that accumulate as the molecule precesses in electromagnetic fields. This means that the sensitivity of these experiments is linear in the time that the molecules spends precessing, which is called the precession time or coherence time. It turns out that the longest precession that that you can reasonably achieve with a molecular beam, even a cryogenic beam like a CBGB, is a few milliseconds. In the ultracold world, coherence times of seconds can be realized with cold and controlled quantum samples. Ultracold molecules therefore have the potential to increase sensitivity to new physics by many orders of magnitude, into the PeV regime.

Reaching the few milliKelvin regime requires more than buffer gas cooling, which inevitably fails around a few hundred milliKelvin. There are several proposed routes, but an extremely promising route is to use what atomic physicists have been using for decades, and has been the workhorse of the ultracold field since then - laser cooling.

Laser-Cooling Molecules

Laser cooling works by using repeatedly scattering photons to apply forces on atoms or molecules. Since photons have momentum, if an atom or molecule absorbs and then re-emits a photon, it will receive a momentum kick - in other words, it experiences a force. However, the momentum kick is very tiny, so typically hundreds of thousands of photon scatters are needed to reduce the velocity of a typical gas-phase atom to the ultracold regime. If you want to learn more about laser cooling, consider takingPhysics 103!

The problem with molecules is that they have internal, mechanical modes - rotation, and vibration - that can be excited when a molecule receives a momentum kick, which will cause the molecule to start rotating or vibrating (or both). Because laser cooling schemes generally address only a single quantum state per laser, exciting these internal modes is effectively loss of molecules. This is a serious problem because if you naively start scattering photons off of a molecule, you about as likely to excite one of these internal modes as you are likely to not excite one - meaning that you can scatter "a few" photons before the molecule is effectively gone. This is so short of "a few hundred thousand" that the naive approach simply doesn't work.

There are a number of very clever tricks that enable laser cooling for certain molecules, which was first demonstrated in 2010. A critical ingredient is using a molecule with electronic structure that is largely decoupled from the vibrational modes of the molecule. This can be achieved, for example, by using an alkaline earth atom with a single ionic bond to an electronegative bonding partner, such as F. Alkaline earth atoms have an s2 valence configuration, so when they bond monovalently, one electron in an s orbital remains localized on the metal. This electron looks very much like it is bound to a single atom, and can therefore be excited via strong optical transitions. Furthermore, the bonding induces some orbital hybridization with excited p states that actually push the electron cloud away from the bonding region, which decouples the optical excitation from the mechanical modes of the molecule.

Figure: Electronic structure of molecules with a single alkaline earth atom with an ionic, monovalent chemical bond. Orbital hybridization induced by the bond push the electron cloud away from the bonding region, which decouples the electronic and mechanical degrees of freedom in both the ground (X) and excited (A) states.

So far, only molecules with this particular electronic structure can be laser-cooled, though there are several examples of laser-coolable molecules that are also sensitive to CP violation. However, there are no diatomic molecules that offer both laser cooling and strong rejection of systematic effects via internal co-magnetometers. Internal co-magnetometers have already shown themselves to be extremely useful for precision measurements in molecules, so it is worth expanding our horizons to find species that offer all of these desirable features.

Polyatomic Molecules

Laser cooling and internal co-magnetometers conflict in diatomics because both features have to come from very specific electronic structure on the heavy atom, and these requirements are at odds. On the other hand, polyatomic molecules, with at least three atoms, offer laser cooling and internal co-magnetometers. Like in diatomics, a heavy metal atom with suitable electronic structure and bonding can provide photon cycling and laser cooling; unlike diatomics, internal co-magnetometers can arise from degenerate vibrational or rotational motions that exist only in molecules with at least three atoms.

Figure: degenerate bending modes in a linear triatomic molecule give rise to states with vibrational angular momentum that can be easily polarized in electric fields. Here the orange atom is a heavy metal center with new physics sensitivity and favorable laser-cooling properties, such as Yb.

Co-magnetometer states that can be easily polarized in electric fields are a generic feature of linear and symmetric top polyatomic molecules that do not depend on the electronic properties of their constituent atoms. Similarly, the laser cooling properties only depend on the electronic properties, and in general not the bonding partners (for suitably chosen ligands.) We can therefore construct molecules with laser-coolable metal centers, such as YbOH or YbOCH3, that can be laser-cooled, have co-magnetometers, and are sensitive to a wide range of new symmetry violating physics, such as nuclear magnetic quadrupole moments and permanent EDMs.

Figure: The metal center (orange) can cycle photons without exciting internal modes, independent of the bonding partners, under a wide range of conditions. Here we show a symmetric top molecule, where the polarizable states arise from low energy rotation of the molecule about the symmetry axis.

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